MARINE ECOLOGY PROGRESS SERIES Vol. 61: 145-157, 1990 Published March 8 Mar. Ecol. Prog. Ser. l Primary and bacterial productivity of tropical seagrass communities in the Gulf of Carpentaria, Australia D. J. W. Moriarty, D. G.Roberts*, P. C. Pollard CSIRO Marine Laboratories, PO Box 120, Cleveland, Queensland 4163, Australia ABSTRACT- Plant and bacterial productivities were compared at different seasons in 3 communities of seagrass in the Gulf of Carpentaria. Australia, in 1985. Seagrass biomass and shoot density were very variable, with no seasonal trend. The gross primary productivity of the seagrasses in each of the communities (Syringodium isoetifoliurn with Cymodocea serrulata in a bay, Halodule uninervis at a river mouth, and Thalassia hernprichii with Cymodocea rotundata on a reef flat) were determined seasonally from rates of lacuna1 gas production. Productivity varied seasonally, being lowest in winter (July).Values ranged from 0.6 to 1.0 g C m-2 d-' for C. serrulata, 0.3 to 8.1 for S. isoetifolium, 0 9 to 3.5 for H. uninervis, 0.2 to 0.4 for T henlprichii and 0.1 to 1.0 for C. rotundata. Gross community primary productivity, measured from diurnal changes in oxygen concentration in the water column, ranged from 3.3 in winter to 9.3 g C m-2 d-' in summer at the bay site, 2.7 to 4 at the river mouth and 3.3 to 8.4 on the reef flat. Bacterial productivity was determined using the rate of tritiated thymidine incorporation into DNA. Most (90 to 95 %) bacterial biomass was produced in the sediment. Between winter and summer, total bacterial productivity (including water column) ranged from 1.0 to 4.8 g C m-2 d-I in the bay, 0.6 to 2.5 at the river mouth and 0.6 to 3.7 on the reef flat. Bacterial productivity averaged 43 O/O (range 10 to 90 %) of gross primary productivity, and thus would account for about half of the primary production if their growth efficiency were 50 %. Animals appeared to have an impact on bacteria at the sediment surface in summer, when specific growth rates and productivity were high, but numbers were low. INTRODUCTION animals have now become possible with the develop- ment of methods for quantifying bacterial production, In pelagic systems, zooplankton may graze at least based on the incorporation of thymidine into DNA 50 % and up to 100 % of primary production (Valiela (Moriarty 1986). Bacterial productivity in temperate 1984). In benthic coastal ecosystems with vascular seagrass beds is linked to seagrass productivity (Mori- plants, however, bacteria are the dominant consumers arty & Pollard 1982, Moriarty et al. 1985).It is difficult to of primary production by the macrophytes, because determine the contribution of bacteria to carbon cycl- most animals cannot digest the fibrous plant material, ing and respiration in sediment in the presence of other which also has a low protein content (Mann 1988). organisms. With the thymidine method, one can esti- Thus where seagrasses are major primary producers, mate bacterial productivity, i.e. the rate at which new they may be important in marine food chains, but bacterial biomass is formed, and then calculate the rate indirectly. Most animals that utilise seagrass produc- of carbon cycling by bacteria if their growth efficiency tion are dependent on decomposition and a detrital is known. food chain or on epiphytic and benthic microalgae in Few studies have been made on productivity of sea- seagrass beds (Moriarty 1976, Klug 1980, Morgan & grasses in the tropical Indo-Pacific region. Recently, Kitting 1984). Quantitative studies on the cycling of however, seagrass beds in Papua New Guinea have carbon from primary producers through bacteria to been studied by Brouns (1985, 1987a, b) and Brouns & Heijs (1986). Production averaged 6.4 g ash-free dry wt Present address: School of Biological Sciences, University of m-' d-' over a year, with little significant seasonal Sydney, NSW 2006, Australia variation in a mixed seagrass bed of 6 species including O Inter-Research/Printed in F. R. Germany 146 Mar. Ecol. Prog. Ser 61: 145-157, 1990 the 5 that were studied in this report (Brouns 198713).In by mangroves and on the seaward side by a coral reef; Indonesia, Lindeboom & Sandee (1989) reported that macroalgae comprised a substantial proporhon of plant gross productivity of a Halodule uninervis community biomass there. The seagrasses Thalassia hemprichii ranged from 2.5 to 4.1 g C m-' d-' and that of a (Ehrenb.)Aschers. and Cymodocea rotundata Ehrenb. et Thalassia hemprichii community ranged from 1.2 to Hempr. ex Ashers. were present (described by Poiner et 4.7 g C m-' d-' in October Net productivity ranged al. [l9871as site group C).The sediment was a muddy silt from 0.06 to 1.1 g C m-' d-l. Thus plant productivity in with some coral rubble, 4 to 7 cm deep on a hard these tropical communities is high and we would calcareous substrate. At each study site a net or cage of 4 expect bacterial productivity to be high also. to 6 m2,with a mesh of 80 to 100 mm, was used to protect We selected 3 communities in the GuIf of Carpen- research workers from crocodiles and sharks. taria, Australia, for studies on relationships between Studies were carried out in January, May, July and primary and bacterial productivity. The productivity of October 1985. Water temperatures were: January, each community, and of seagrasses and bacteria in mean 32 "C, maximum 34 "C (38 "C on the reef flat); small sections of the communities, were studied to May, mean 30 "C, maximum 31 "C; July, mean 23 "C, provide some insight into carbon cycling during each maximum 24 "C; October, mean 29 "C, maximum season, especially in the sediment. 33 "C. The highest temperatures occurred over the reef flat. The light intensity in the seagrass beds was meas- ured with a submersible integrating light meter (Bio- MATERIALS AND METHODS spherical Instruments Inc., Model QSI-140). Maxima were 1500 pE mW2S-' at the river mouth and bay sites Study sites. The study sites were at Groote Eylandt, a and 2000 at the reef flat in January; 1200 in May; 900 in continental island with similar geomorphology to the July and 2400 in October. Day lengths (light intensity mainland nearby; the climate is monsoonal with rain of > 200 1iE m-' S-') were l1 h in January, 10 h in May, occurring mostly between December and Apnl (Fig. 1). 9.7 h in July and 10.5 h in October. All temperatures and oxygen concentrations in the water column and bottles were measured with a sub- mersible electrode that produces oxygen at the same rate at which it is consumed, and therefore does not require the water to be stirred (Leeds and Northrup, Model 8500). Biomass and shoot density. Six replicate cores (10 cm diameter) of seagrass shoots, rhizomes, roots and sediments were collected to a depth of 12 cm at the first 2 sites and to the full depth of the reef flat (5 to 7 cm). Shoots were counted and then the above-ground plant material was removed and separated into sea- grass species. Plant material (rhizomes and roots) was sieved out from sediment and separated into species. All plant material was dried at 105 "C and weighed. Primary productivity. The die1 curve procedure Fig. 1 Location of study sites at Groote Eylandt, Gulf of described by Odum & Hoskin (1958) was used to deter- Carpentaria mine g.ross community productivity and respiration from changes in oxygen concentration in the water The first sampling site was near a river mouth. It was a flowing over the seagrass bed. Techniques for deter- monospecific stand of Halodule uninervis (Forsk.) mining productivity that are based on measurements of Aschers. in Bolssier on a sandy sediment in a community changes in oxygen concentration in the water around of H. uninervis and Halophila ovalis (R. Br.) Hook. F. seagrasses give good estimates of the contribution of inshore and Syringodium isoetifoLium (Aschers.) Dandy algae as well as the seagrasses (see 'Discussion', and and Cymodocea serrulata (R. Br.) Aschers. et Magnus Lindeboom & Sandee 1989). offshore (site group E described by Poiner et al. 1987). To determine the relative contributions of benthic The second sampling site was a bay with mostly S. and pelagic organisms to the uptake of oxygen from the isoetifoliurn and some C. serrulata on a silty clay sedi- water column, oxygen changes were measured in ment (site group F described by Poiner et al. 1987) There enclosed dupl.icate samples of the water column were no macroalgae present at that site. The third together with and separate from the benthos. Samples sampling site was a reef flat fringed on the landward side of the wa.ter column were enclosed in 4.5 1 clear glass Moriarty et al.: Productivity of tropical seagrass communities 147 bottles and incubated in situ. Cores (100 mm diameter) added to preserve the samples, which were then fil- containing sediment, macroalgae and seagrass were tered at the laboratory, and washed with trichloroacetic collected and clear acrylic chambers (2 1 capacity) acid as described by Pollard & Moriarty (1984). placed over them. They were incubated underwater in Bacterial productivity was determined in the fine, the seagrass bed. The water in each enclosure was flocculent material that settles onto the seagrasses and stirred before oxygen concentrations were measured. sediment in calm conditions, but is easily resuspended The photosynthetic rates of the seagrasses were by disturbance. Covers (100 mm diameter), with 2 1 determined with the lacunal gas technique (Roberts & chambers attached, were carefully placed over the sea- Moriarty 1987).